1. Field of the Invention
The present invention relates to a memory cartridge. More specifically, the present invention relates to a memory cartridge which is attachable to and detachable from a main unit of a personal computer or a home video gaming machine which includes a computer and is loaded in the main unit when used.
2. Description of the Prior Art
Home personal computers or the gaming machine which includes a computer called the "Nintendo Entertainment System" (trade mark) manufactured and sold by the assignee of the present invention and the one called the "MSX" (commodity name) use an external memory cartridge in which a game or educational program or the like is written in advance is used, and the computer is enabled by loading such external memory cartridge in the main unit. The memory cartridge includes a non-volatile memory (for example, ROM) for storing program data and/or character data for display.
When a central processing unit (CPU) located in the main unit of the computer accesses the ROM of the memory cartridge, the maximum number of accessible addresses, that is, address space is limited by the performance (number of bits) of the central processing unit, and therefore the usable memory capacity of the ROM comprised in the memory cartridge is also limited naturally. For example, in the above-described "Nintendo Entertainment System", only a 256K-bit ROM can be used for program and a 64K-bit ROM for characters at a maximum. Thus, the maximum number of program steps is limited to the maximum address space accessible by the central processing unit, and therefore when such a computer is used as a gaming machine, for example, the length of story of the game, the extension of variation in the game, the number of display scenes and the number of characters capable of being displayed are limited.
One prior art approach which was proposed to eliminate such an inconvenience disclosed in is, for example, the Japanese Patent Laid-Open No. 112352/1984, laid open on June 28, 1984, which corresponds to the U.S. patent application Ser. No. 261,301, now U.S. Pat. No. 4,368,515.
In the above-identified prior art, an address from the central processing unit installed in the main unit of the gaming machine is given to a plurality of memory chips as a common address input, while that address is decoded by an address decoder. When a specific address is outputted by the central processing unit, the address decoder outputs a signal, and in response to the signal, a flip-flop or a latching circuit is operated. From the flip-flop or the latching circuit, a chip select (CS) signal for selecting a chip corresponding to that specific address is outputted, and the chip select signal enables the corresponding memory chip. Accordingly, the memory area designated by the address of the selected memory chip can be accessed by the central processing unit.
The above-described prior art has an advantage that the memory capacity can be expanded without increasing the address ports from the central processing unit, but leaves the following problem to solve.
With the recent advance in the semiconductor technology, the degree of integration of integrated circuit chip is being more and more increased, but the above-described prior art cannot accommodate for such a one-chip, large-capacity memory. Because, output of the flip-flop or the latching circuit is used as a chip select signal, and such a chip select signal can only select enabling or disabling on a chip basis, and cannot perform enable/disable of the specific area in the one-chip memory. In other words, in the prior art, an arbitrary chip of the memory chips respectively having the number of addresses accessible by the central processing unit can be enabled to designate an address, but banks respectively having addresses accessible by the central processing unit of a one-chip memory having addresses of a number larger than the address space accessible by the central processing unit cannot be selected or addressed. Accordingly, by the prior art, the benefit of the up-to-date semiconductor technology cannot be given, and the ratio of the rise in cost to the increase in memory capacity is large, eventually resulting in a high price.
In addition, various other methods of changing over the memory banks have been proposed, but any of them does not relate to the memory cartridge intended by the present invention.